Abstract

The Boogardie Synform, at Mt Magnet, in the NE Yilgarn Craton of Western Australia, is difficult to explore due to structural complexity in the basement, a variably truncated, residual regolith beneath a complex, transported overburden and high geochemical backgrounds from numerous mineralized settings. The distributions of Au and associated elements in the regolith around the Stellar and Quasar deposits, within the Boogardie Synform, have been studied to identify optimum sample media and an analytical suite for continuing exploration of this district. Regolith–landform relationships and geochemical dispersion in the district were established by mapping the regolith of an area of 25km2 around the mines and investigating drill cuttings and pit exposures.

Mining and drilling at both sites revealed a partly eroded lateritic regolith incized by a fluvial palaeochannel, all concealed beneath 5–10 m of polymictic colluvium–alluvium. The sediments have all undergone post-depositional weathering. The Au mineralization is poor in As and Sb, but there are weak anomalies in Bi and Pb in the upper part of the weathered basement. There is no expression of the mineralization in the palaeochannel sediments or colluvium, nor between nearly adjacent layers within the colluvium and its overlying lag. Comparison of data from samples taken from the top of the basement with those from the unconformity, or interface, between basement and colluvium, indicates lower order but broader (200 m) and more consistent anomalies in the interface for Au, Bi and Pb. Useful anomalies can be improved using additive indices of Bi, Pb and Zn. The interface sample is, therefore, the preferred sample medium in this area of buried, truncated regolith, except where there are palaeochannels. Where there has been less regolith truncation and a buried, ferruginous duricrust is present, as at Stellar, duricrust is the optimum sample medium.

Elevated concentrations of Cr, Ni, Cu, V and, possibly, Zn in the basement regolith indicate mafic–ultramafic rocks. The saprolite and mottled zone on the basement rocks have a significantly greater Cr/Fe ratio than the transported materials. Regolith differentiation can be improved by robust canonical variate analysis; the most useful elements appear to be Al, Fe, Ni, Cr, Ga, Y, Zn, Th and Cu.

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